1. Field of the Invention
The present invention relates to digital tone detector circuits and more particularly to a digital tone detector circuit providing reduced sidelobe and harmonic responses.
2. Description of the Prior Art
Analog tone detectors utilizing analog tone filters have been used for many years in communication receivers to provide selective call signaling capability. Such detectors, while they have proven suitable for use in many applications, have become less desirable for use in new applications because of a number of problems, such as cost, and inventory requirements for replacement and re-coding of the selective call addresses. A number of digital microcomputer based detectors have been devised to replace the analog tone filter designs to alleviate the aforementioned problems. Many of these microcomputer based detectors have utilized cross correlation detector implementations for detecting the possible presence of periodic signals of known frequency that may be buried in noise, as would exist in weak signal conditions. The cross correlation detection technique utilized up to now is, of course, well known; however, practical implementation of this method has up to now necessarily entailed performance compromises. In particular, many of these early detector implementations have had high susceptibility to false detection due to signals at harmonics of the desired signal. Most of the early detectors have also generally exhibited excessive sidelobe responses that limited the "detection threshold", the limit at which signals can be detected in noise. Other of the early detectors exhibited responses that were dependent upon the phase of the input signal which further limited the "detection threshold" and further increased the susceptibility to falsing. In order to overcome the falsing problems of these early detectors, the detector sensitivity had to be reduced as compared to some of the analog active filter detectors which resulted in reduced receiver signaling sensitivity. These early microcomputer based detectors required high clock rates (3 to 8 MHz), which resulted in increased power consumption as compared to the earlier active filter detectors.
One of the earliest phase independent implementations of a microcomputer based detectors is described in U.S. Pat. No. 4,302,817 to Labedz which is assigned to the assignee of the present invention. A simplified block diagram of the Labedz tone detector is shown in FIG. 1A which is largely based on the earlier prior art analog phase independent correlation detector shown in FIG 1B. The Labedz detector greatly simplified the design of a digital tone detector. In the place of the analog tone input signals, the Labedz detector utilized limited data input signals. The analog sin (Wt) and cos (Wt) reference signals were converted to squares wave reference signals SQS (Wt) and SQC (Wt). The limited data input signals and square wave reference signals SQS (Wt) and SQC (Wt) had one of two values, namely +1 or -1, to ease the digital processing. In the Labedz detector, the analog multiplications 120 and 122 in FIG. 1B were replaced by simple Boolean multiplications 100 and 102 in FIG. 1A. The integrators 124 and 126 utilized in the analog design of FIG. 1B were replaced by summers, 104 and 106 in FIG. 1A. The squaring functions 128 and 130 of FIG. 1B required in the analog design were replaced by absolute value circuits, 108 and 110 in FIG. 1A. The outputs of the squaring functions were linearly added 132 in the analog design of FIG. 1B, whereas the outputs of the absolute values circuits are digitally summed in summer 112 of FIG. 1A. The analog square root function 134 required in the analog design of FIG. 1B was replaced by a simple divide by two function 114 in the Labedz detector of FIG. 1A. In actual practice, the divide by two operation was generally omitted, since the output, K.sub.n was compared to a detection threshold, K.sub.th, which would take into account the divide by two factor. When the detector output K.sub.n exceeded the detection threshold K.sub.th, as indicated for decision circuit 116 of FIG. 1A, a logical one detect output was generated, indicating the detection of the desired tone; otherwise, the output of the decision circuit 116 remained at a logical zero. A detection in the digital implementation thus corresponds to the output voltage V.sub.o exceeding the reference voltage V.sub.r in FIG. 1B.
While not specifically shown in FIG 1A, the input and reference signals were sampled periodically, and at each sampling instant, the input sample value (+1 or -1) was multiplied by the reference level (+1 or -1) and added into the correlation total. The correlation or observation window in time was then N.times.T.sub.s, where N was the number of samples taken during a predetermined sampling period and T.sub.s was the time interval between samples within the predetermined time period. Further modifications to the decoder of FIG. 1A were required to make the tone detection process pseudo continuous. A tone presence decision was made only once at the end of the correlation window, after every N samples have been taken. For very narrow bandwidths, such as 12 Hz, the tone presence decision would be made only once every 150 msec, as compared to the analog detector where decisions were essentially made after every cycle. In most tone detection applications, detection decisions are needed more frequently. In order to make the decision process somewhat continuous and analogous to the analog tone detection process, the observation window of the early digital decoders was broken down into a number of correlation subwindows, for example M subwindows, and a correlation sum was computed after each subwindow, accumulation or every N/M samples. The total correlation sum was then computed by summing the M subwindow accumulations together. In this way, a new correlation sum was computed after each subwindow. The total summation was then very easy to compute, as the oldest subwindow total was subtracted from the newest subwindow accumulation and the total was then added to the present correlation sum.
While the Labedz tone detector design of FIG. 1A proved to greatly simplify the implementation of a tone detector as compared to the analog designs, the sensitivity of the tone detector was less than the analog tone filter decoder due to increased sidelobe responses and increased harmonic responses, particularly third harmonic responses. As a result detector sensitivity was compromised compared to the prior art analog tone detectors. The sidelobe responses were due largely to the use of a rectangular observation window for the Labedz decoder, as shown in FIG. 2, and the response at the harmonics resulted from the use of rectangular SQS (Wt) and SQC (Wt) reference signals at the tone frequency of interest, as shown in FIG. 4.
Muri, in U.S. Pat. No. 4,513,385 issued Apr. 23, 1985, entitled "Apparatus and Method for Suppressing Side Lobe Response in a Digitally Sampled System" which is assigned to the assignee of the present invention, realized one solution to a portion of the aforementioned problem. Muri described the use of a substantially rectangular observation window T1, as shown in FIG. 3, which omitted sampling of the input signal during a predetermined time interval T2, which resulted in a significant decrease in the undesired side lobe responses. The Muri window function, while suitable for use with tone signaling protocols, such as the ZVEI and CCIR tone signaling protocols having a limited number of tones, such as twelve tones evenly spaced between 350 Hz and 3100 Hz, was found to be unsuitable for decoding numerous closely spaced tones, such as found in the Motorola Quik-Call II signaling protocol, which utilizes sixty tones spaced at 2.77% frequency intervals between 280 Hz and 3100 Hz.